Fuel cells are electrochemical devices that produce direct current (DC) electricity by the reaction of a fuel with an oxidant, typically producing byproducts of heat and water. Common fuels are hydrogen, methanol, and carbon monoxide; however, carbon monoxide can only be used as a fuel in high-temperature fuel cells operating at temperatures >400° C. The most common oxidant is oxygen, either in a relatively pure form or from air. Fuel cells contain an anode, a cathode, and an electrolyte barrier between the anode and cathode. The fuel is introduced at the anode and the oxidant is introduced at the cathode. The electrolyte barrier, commonly referred to as a membrane-electrode assembly or MEA, is an ionically conductive thin barrier that is relatively impermeable to the fuel and oxidant, and is electrically insulating. Known fuel cell designs and operating principles are described in, for example, The Fuel Cell Handbook, 7th Edition (2004) published by the US Department of Energy, EG&G Technical Services under contract DE-AM26-99FT40575.
Many configurations of fuel cell systems are known. Portable fuel cell systems are based on several different types of fuel cells, including proton-exchange membrane fuel cells (PEMFC) that operate at temperatures less than 85° C. and that use high-purity hydrogen as the fuel; PEMFCs that operate at temperatures in the 135° C. to 200° C. range and that use hydrogen-rich reformate as the fuel; direct methanol fuel cells (DMFC) that operate at temperatures less than 85° C. and that use methanol as the fuel; and solid oxide fuel cells (SOFC) that operate at temperatures in the range of 500° C. to 900° C. and that use hydrogen-rich reformate as the fuel.
Protonex Technology Corporation of Southborough, Mass. is developing a family of PEMFCs that operate at temperatures less than 85° C. and that use high-purity (>99%) hydrogen that are designed to generate from 30 W to 500 W. Protonex has teamed with Millennium Cell to couple a sodium borohydride hydrogen generator to the fuel cell. The Millennium Cell hydrogen generator produces high-purity hydrogen by the catalyzed reaction of water with dissolved sodium borohydride. Drawbacks of the sodium borohydride hydrogen generator include high cost of the sodium borohydride and the relatively short shelf life of the required aqueous solution of sodium borohydride.
Voller Energy of the UK offers a 100 W portable fuel cell system (Model VE100 V3) that incorporates a low-temperature (<85° C.) PEMFC that uses pure hydrogen delivered from a small canister. The system's drawback is that hydrogen fuel is very expensive and is considered hazardous, thereby subjecting it to shipping restrictions. Moreover, one canister provides only three hours of operation at 100 W.
IdaTech LLC of Bend, Oreg. has demonstrated a prototype 250 W fuel cell system based on a low-temperature (<85° C.) PEMFC that includes a methanol/water reformer. The IdaTech reformer also includes a hydrogen-selective membrane that yields a stream of high-purity hydrogen for fuel. The drawbacks of this design are its inherently high cost and complexity.
Portable DMFC products sold under Model Nos. A50 and C20-CP are made by Smart Fuel Cell AG of Brunnthal-Nord, Germany. The Model A50 is an autonomous battery charger rated at a maximum of 50 W and requiring a 12 V lead-acid battery. The Model C20-CP is a hybrid DMFC/Li-polymer battery that delivers a maximum of 20 W continuous and 36 W peak power. Both models include gas-liquid separators, the use of methanol/water mixtures with very low methanol concentrations, a water recovery/recycle subsystem and a microprocessor and methanol sensor combination to monitor, measure and control the methanol concentration. The chief drawbacks of such DMFC products is that they are very complex and expensive.
There is therefore a need for portable fuel cell systems that are affordable and that use simple and inexpensive liquid fuels. The present invention addresses these needs.